1. Field of the Invention
The present invention relates to a nitride semiconductor element formed of a semiconductor layer where a nitride semiconductor made of AlxInyGa1-x-yN (0≦x, 0≦y, 0≦x+y<1) is layered, and to a manufacturing method for the same, and in particular, to a semiconductor light emitting element such as a light emitting diode (LED) or a laser of which the light emitting efficiency is improved, as well as to a manufacturing method for the same.
2. Description of the Related Art
A light emitting element using a nitride-based semiconductor such as gallium nitride makes light emission of ultraviolet light, blue light, green light and the like possible with high efficiency and low consumed power, and makes miniaturization possible, increasing the strength against mechanical vibration and the like, with the advantage of long lifespan, high reliability and the like, and therefore, is widely applicable to a variety of fields. In particular, applicability of a light emitting element to a large size display, a traffic signal, the backlight source of a cellular phone and the like has significantly become widespread.
It is important, in a light emitting element using a nitride-based semiconductor, to enhance the light emitting efficiency so that light generated in the active layer can be emitted to the outside for effective use. From such a point of view, a conductive film having translucency is required as an electrode, and ITO (compound oxide of In and Sn), SnO2, ZnO, for example, are utilized. In particular, ITO is an oxide conductive material where indium oxide contains tin and has a low resistance and a high translucency, and therefore, is appropriate for a translucent electrode and the like.
FIG. 1 shows an example of an LED utilizing such a translucent electrode. The LED has a structure where an n-type GaN layer 2, an InGaN light emitting layer 3 and p-type GaN layer 4 are epitaxially grown in sequence on a sapphire substrate 1 via a buffer layer. In addition, portions of InGaN light emitting layer 3 and p-type GaN layer 4 are selectively etched and removed so as to expose n-type GaN layer 2. An ITO layer is formed on p-type GaN layer 3 as a p-type translucent electrode 5, and a bonding pad of a p-side electrode 7 is further layered on top of this. In addition, an n-side electrode 8 is formed on n-type GaN layer 2. These electrodes are formed by depositing metal such as Al, Au, In or the like. In such a structure, a current that has been injected via p-side electrode 7 is uniformly diffused in the ITO layer, which is p-side translucent electrode 5 having high conductivity, so that the current can be injected from p-type GaN layer 3 to n-type GaN layer 2 so as to emit light. In addition, this light is not blocked by p-side electrode 7, but rather, is transmitted through the ITO layer so as to be emitted to the outside of the chip.
However, such a nitride-based semiconductor light emitting element has a problem with high contact resistance in the electrode portions. This is because the band gap of GaN is as wide as 3.4 eV, making ohmic contact with the electrodes difficult. As a result of this, the contact resistance of the electrode portions becomes high, enhancing the operation voltage of the element, which causes the problem of high consumed power and large amount of heat emission.
On the other hand, light emitting efficiency is also poor. This is because the index of refraction of GaN is as great as approximately 2.67, and therefore, the critical angle is 21.9 degrees, which is extremely small. That is to say, light that has entered at an angle greater than this critical angle relative to the normal line to the main light emitting surface cannot be emitted to the outside of the LED chip, but rather, is confined therein. As a result of this, it is difficult to improve the external quantum efficiency and gain a greater light emitting power.
Here, in the case where the surface of the p-type GaN layer, which is the main light emitting surface, is processed so as to have an uneven form, the above described problem can be dealt with. However, it becomes necessary for the p-type GaN layer to have a certain thickness, in order to create the uneven form. Also, in the case where a thick p-type GaN layer is formed while doping an impurity with high concentration in order to reduce as much as possible the contact resistance with the electrode, a new problem arises, wherein surface coarseness of the crystal surface occurs.
Japanese Unexamined Patent Publication No. 2000-196152 discloses an LED element where the light emitting efficiency has been improved by providing unevenness on the main light emitting surface in order to solve the above described problem. In the LED disclosed in Japanese Unexamined. Patent Publication No. 2000-196152, the interface between the ITO layer, which is a translucent electrode, and the p-type GaN layer, which is: a p-type semiconductor layer, is made uneven so as to provide the configuration that makes it easy for the light reflected from this interface to be emitted to the outside, improving the emitting efficiency. Concretely speaking, the surface of the p-type GaN layer is processed so as to be uneven, and a translucent metal electrode or a translucent electrode is provided on top of this.
However, in the case where the surface of a GaN layer is processed so as to be uneven, a problem arises wherein the epitaxial growth layer is damaged in a manner where this portion cannot emit light. In addition, when the p-type GaN layer is too thin, it allows the process for forming the unevenness to damage the active layer and the n-type GaN layer, and therefore, the GaN layer must be a thick film, in order to avoid such a problem. On the other hand, though there is a method where the surface of the p-type GaN layer is made to be flat and the surface of the p-side translucent electrode is processed so as to be uneven, it makes the uneven surface too close to the main light emitting surface, making this uneven pattern visible easily from the outside, and the problem of poor appearance arises.
In addition, in a so-called face-up structure where a p-type electrode is provided on the main light emitting surface, it becomes necessary to provide an n-type electrode, making the main light emitting surface smaller, and in addition, it is necessary for a pad electrode to be provided on the p-type electrode, causing light to be blocked by this portion, which prevents light emission, and the problem of poor light emitting efficiency arises.
In addition, in the structure where a conductive oxide film represented by ITO is used as a translucent conductive layer, it is necessary to increase translucency, and furthermore, in the structure where the conductive oxide film is provided so as to make contact with a semiconductor layer, it is also necessary to enhance ohmic properties. However, it is difficult to increase translucency and enhance ohmic properties at the same time, and a further increase in the performance of the conductive oxide film is required.